I think he explicitly said it was a diversion and with a mess that big they won't even to bother to count body parts (in other words they would've thought he was also dead and would've closed the case and not kept hunting him).
shareThis the same guy
https://moviechat.org/tt0111257/Speed/58c75e426b51e905f67e3f6c/What-was-the-most-evil-thing-Howard-Payne-did
https://moviechat.org/tt0111257/Speed/58c75e416b51e905f67e3f36/Who-was-more-evil-Howard-Payne-or-the-Joker-from-The-Dark-Knight
https://moviechat.org/tt0111257/Speed/58c75e426b51e905f67e3f6c/What-was-the-most-evil-thing-Howard-Payne-did
https://moviechat.org/tt0468569/The-Dark-Knight/5b5c8d3ab747be00144c42ed/What-do-you-think-was-the-most-evil-thing-that-the-Joker-did?reply=5b5c9a17b747be00144c4346
And several of newer ones from this site that will give you Deja Vu.
Now excuse me, but... Holy Howard Payne/Joker/movie psychos and repeated posts obsession, Batman!
Because he was surrounded by cops and wanted to get away with the money. Annie was simply a hostage, he didn't care whether she lived or not.
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